The development of rechargeable alkali metal-ion batteries requires use of high quality salts. These salts should meet such requirements such as: providing high ionic conductivity over wide temperature range; being electrochemically stable relative to cathode and anode materials, especially at the fully charged state; being capable of passivating current collector material, such as aluminum, of the cathode at high potentials; being able to assist the formation of a stable solid electrolyte interphase (SEI) with carbonaceous anode materials; having high solubility at low temperatures; and being thermally stable at high temperatures. Among many commercially available lithium salts, only few are found to satisfy the above requirements. These salts include lithium hexafluorophosphate (LiPF6), lithium perfluoroalkyl-substituted fluorophosphates, as detailed in U.S. Pat. Nos. 6,210,830 and 6,423,454, lithium tetrafluoroborate (LiBF4), and recently developed lithium bis(oxalate)borate (LiBOB), as detailed in Patents DE 19829030 C1 and U.S. Pat. No. 6,506,516. It is noted that all these salts either contain phosphorus or contain boron. Extensive spectroscopic analyses have revealed that the SEI on the surface of carbonaceous anode in lithium-ion batteries must contain molecular moieties of halogen and phosphorus, or halogen and boron. These analyses suggest that the unique properties of these salts in lithium-ion batteries are associated with the presence of phosphorus and boron.
LiBF4 has been used as a conducting salt for electrolytes in both primary cells and rechargeable cells. The electrolytes in these batteries are non-aqueous solutions of LiBF4 in organic solvent, e.g. in dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, ethylene carbonate, propylene carbonate, other carbonates, or mixtures of the solvents mentioned. However, LiBF4 is relatively inefficient in facilitating the formation of stable SEI on the surface of graphite. Additionally, an electrolytic solution containing LiBF4 salt has relatively low ionic conductivity because of the tight ion pairing between Li+ cations and BF4− anions.
Several prior art references teach methods for preparing highly pure LiBF4 by a means of the reaction of BF3 and LiF in non-aqueous media. These have all met with limited success.
SU 1013405 describes the preparation of LiBF4 in tetrahydrofuran (THF) by reacting LiF with BF3 in yields of from 86 to 89%. The product is isolated by concentrating the THF solution. This generally gives a product containing considerable amounts of residual THF. To remove residual THF, drying in vacuum at from 70 to 80° C. for from 10 to 15 hours is proposed.
JP-A 56145113 describes a process for preparing LiBF4 by reacting LiF with BF3 in non-aqueous organic solvents in which LiBF4 has good solubility and which can form complexes with BF3. Non-aqueous organic solvents mentioned are tetrahydrofuran, dimethoxyethane, ethyl acetate and propylene carbonate. After the reaction of LiF with BF3, impurities are filtered off. LiBF4 is crystallized from the filtrate by saturating the solution with BF3. With the solvent, BF3 forms a complex in which LiBF4 has low solubility, and the product crystallizes.
U.S. Pat. No. 6,537,512 describes a method for preparing LiBF4 by reacting BF3 etherate and suspending LiF in dimethyl ether solution to produce LiBF4. As the solubility of LiBF4 is low in dimethyl ether, the formed LiBF4 can be easily separated off by traditional filtering methods. However, LiF being insoluble in dimethyl ether is present in the prepared LiBF4.
Lee et al. describes in Journal of the Electrochemistry Society 145(8), 2813-2818 (1998) that LiCl is more soluble in dimethyl ether than LiF, and that LiCl dissolved in the solution can easily form highly soluble 1:1 mole ratio complex with a variety of boron-based organic anion acceptors. No effort is reported as to the separation of resulting complex as a pure salt nor is BF3 described as the anion acceptor.
In view of the failure of any known salt to adequately satisfy the requirements for an alkali metal-ion battery salt, there exists a need for a new salt produced by a method of production that yields a high purity salt in scaleable quantities.